In recent years, services for sharing content data have become popular among specific users such as SNS (social networking service). Such a service accumulates shared content data to be shared, for example, among specific users, in a cache server or the like, and allows user terminal devices to refer to the shared content data in the cache server.
Incidentally, a conventional cache server deletes content data having a smaller number of references if shortage of the memory capacity occurs. Therefore, there may be cases where shared content data supposed to be shared by terminal devices is deleted on the cache server.
In this case, the cache server obtains shared content data to be shared, by searching for the data among other servers after receiving access from a terminal device until receiving a reference request. As the search method, for example, a method is known that narrows down servers to be searched for so that searching is performed only on servers that store desired files.